The invention relates to a method for operating a spark ignition engine, in particular in a utility vehicle wherein at least one inlet valve is closed very late or very early and air compressed by a charger is supplied to the engine.
Usually diesel engines are used in utility vehicles, which, depending on the design with high compression ratios providing for a comparatively high thermodynamic efficiency level have generally a good operating efficiency. However, the injection systems of the diesel engines and other components are expensive. Furthermore, their high level of pollutant emissions, in particular soot emissions in the full load operating mode, is also disadvantageous.
Spark ignition engines can be used, given a corresponding design, with an air ratio of λ=1 or higher, that is to say in a lean operating mode. The resulting exhaust gases are virtually soot-free compared to the diesel engine. However, the operating efficiency of spark ignition engines is limited by a large number of thermodynamic peripheral conditions. In view of the tendency of motor fuel to experience premature auto-ignition (knocking), the geometric compression ratio must not exceed a specific limit value and is significantly below that of a diesel engine. The thermodynamic efficiency is correspondingly limited.
In order to improve the efficiency it is known to operate a spark ignition engine according to the so-called Miller method. In this method, the spark ignition engine is given a compression ratio which is higher than in a normal spark ignition engine. In order to compensate for the resulting tendency to knock, the inlet valves of the cylinders are closed very early or very late, that is to say significantly before or significantly after the assigned bottom dead center BDC=540°. As a result, intake of fresh combustion air is either interrupted early or else part of the taken-in quantity of air is pushed back into the intake manifold. As a result, the respective cylinder is only filled incompletely. Despite the increased geometric compression, the combustion air pressure and temperature levels are therefore reduced whereby knocking can be avoided.
In a particular development of the abovementioned method as it is known for example from DE 199 50 677 A1, in the partial load range, a relatively high compression ratio is provided. By adjusting the opening time of the inlet and also of the outlet valves in the full load range, the compression pressure of the combustion gas can be kept relatively low in spite of the increased compression ratio. In addition, supercharging can be provided. A lean operating mode is optionally suggested for the low and partial load ranges. In the throttled state, that is to say in the lower load range, exhaust gas recirculation can also be performed.
A spark ignition engine which is operated in this way can be operated reliably only to a limited degree, in particular in the full load operating mode. The pistons and components which conduct exhaust gas in the region of the outlet valves are subject to high thermal loading. In order to reduce the thermal loading, enrichment of the fuel/air mixture is usually performed in the full load operating mode, which leads to disadvantages in terms of fuel consumption and increased emissions of pollutants.
In particular in view of the last-mentioned disadvantages, spark ignition engines which are operated in this way are suitable for utility vehicles only to a limited extent. Utility vehicles are operated, in comparison to passenger cars, over much larger periods, under high or even under full load. In order to still achieve a sufficiently long service life of the internal combustion engine, structural measures and/or operating parameters are to be provided which limit the thermal-mechanical loads on the various engine components. The usual thermal limiting through enrichment of the fuel/air mixture at full load leads, particularly in utility vehicles with high full load operating periods, results in inadequate operating efficiency so that the potential for low emissions of pollutants is not utilized.
It is the object of the present invention to provide a method for operating a spark ignition engine with improved operating efficiency.